The long-term goal of the proposed study is to determine if the neuromuscular junction of fast-twitch red, fast-twitch white and slow twitch intermediate muscle fibers is under neurotrophic influence, and to correlate any changes observed in the neuromuscular junction with differences in muscular physiology and morhpology. This will be assessed by first establishing any differential characteristics of the neuromuscular junction that might exist in fiber types and then by looking at neuromuscular synaptic physiology after cross-reinnervation and denervation. The question of the extent of the influence of specific "fast-twitch red, fast-twitch white or slow-twitch intermediate nerve" has on synaptic physiology will be further investigated by hyperinnervating a normally functioning muscle with a supplementary foreign nonfunctional (as far as transmission of an impulse is concerned) nerve supply. These experiments should provide information that will assist in determining the importance of the neuromuscular junction in qualitatively regulating the properties of skeletal muscle fibers. This assessment will be based on morphological, chemical and physiological cata.